1. Field of the Invention
The invention relates to a voltage-controlled capacitor circuit and related circuitry, and more particularly, to a metal-oxide semiconductor (MOS) varactor and a diode varactor to form a capacitor with preferred a characteristic of voltage-controlled capacitance and other circuits with related applications.
2. Description of the Prior Art
In modern information business, all kinds of data, information, video, and so on are all transmitted electronically; therefore, a processing circuit for dealing with electronic signals becomes the most important foundation of modern information business. For example, in common information systems (such as a personal computer), a global clock is required to coordinate all digital circuits in the system, so an oscillator for generating clock is an indispensable circuit block for modern digital circuits. In addition, to synchronize circuits with different clocks, phase loop lock (PLL) circuits are needed, and a precise voltage-controlled oscillator (VCO) is essential for the PLL to generate different frequencies of signals. Furthermore, in some precise filters, resister-capacitor (RC) filters, in which filter frequency can be adjusted, are utilized frequently.
With filter characteristics of an RC filter and the oscillation characteristic of an LC oscillator, it is possible to adjust each of them by modifying the capacitance value. Please refer to FIG. 1. FIG. 1 is a schematic diagram of a prior art VCO10. The VCO10 has a pair of coupled MOS transistors T1, T2 to form an oscillation circuit, and voltage Vd biases the VCO10. The gate electrodes of T1 and T2 are input ends of the VCO, and are connected electrically with nodes NP2 and NP1 separately. The drain electrodes of T1, T2 are output ends of the VCO, and are electrically connected to Np1 and Np2 separately. A current source Ip0 that is electrically connected to the source electrodes of transistor T1 and T2 provides current for the circuit. And a pair of coupled diodes Dp1 and Dp2 serve as varactors, and their cathodes are electrically connected to nodes Np1 and Np2. Their anodes being controlled by the voltage Vc0 makes these two diodes Dp1 and Dp2 reversed-biased, and results in a depletion region in the PN-junctions of each of them. This will make Dp1 and Dp2 form an equivalent capacitor between each of their anodes and cathodes. The capacitors provided by the diode Dp1 and Dp2 can form an LC tank with a pair of coupled inductors Lp1 and Lp2. The capacitor provided by the diode Dp1 and the inductor Lp1 can be regarded as the load of the transistor T1. If the transistor T1 receives an input oscillation signal from its gate electrode and outputs the signal through Np1 to the load, it is equivalent to modifying the phase of the input oscillation signal. The input oscillation signal enters the gate electrode of the transistor T2 through node Np1 after being phase-modified by the transistor T2 again, and from node Np2 it will feedback to the transistor T1 once again. According to the feedback mechanism mentioned above, it will produce a periodical oscillation signal at nodes Np1 and Np2.
Since the VCO10 deploys the LC circuit to generate a resonant signal, the frequency of the resonant frequency is proportional to 1/√{square root over ((L0·C0))}; where L0 is the inductance value of the inductors of Lp1 and Lp2, and C0 is the equivalent capacitance value of the diodes Dp1 and Dp2. In VCO10, the diodes Dp1 and Dp2 serve as voltage-controlled varactors. The capacitance of the varactors can be simply modified by changing the voltage across the anode and the cathode. Furthermore, changing the voltage means changing the resonant frequency of the VCO, and this will achieve the goal of voltage-controlled frequency. Please refer to FIG. 2A and FIG. 2B. FIG. 2A and FIG. 2B are schematic diagrams of the relationship between the voltage and the equivalent capacitor across the anode and the cathode when a diode is used as a varactor. When a diode D0 acts as an equivalent varactor, with a capacitance of Ca0 between Na1 (cathode) and Na2 (anode), a relationship between Va12 (the voltage across cathode and anode) and Ca0 is shown in FIG. 2A; where the X-axis is the voltage of Va12, and the Y-axis is the capacitance of Ca0. On the other hand, FIG. 2B shows a relationship between the reciprocal and square root of the capacitance of Ca0 and the voltage of the Va12. When the diode D0 is applied into LC-VCO, it is better for the circuit designer to linearly voltage-control the resonant frequency of the VCO. Since the resonant frequency is proportional to 1/√{square root over (Ca0)}, it represents that a linear relationship between the controlled-voltage and 1/√{square root over (Ca0)} would be better. But as FIG. 2B shows, the voltage Va12 across the diode D0 does not show a reasonable linearity with 1/√{square root over (Ca0)}, and would be a problem when designing the circuit. Moreover, that the characteristic of a voltage-controlled-capacitor diode varactor is mostly decided by a semiconductor manufacturing process, which cannot be adjusted by the method of circuit design, makes it inflexible to be applied into circuit design.
In the prior art mentioned above, a diode is used as a varactor. Now that MOS processes have developed, MOS can now be used as a voltage-controlled varactor too. Please refer to FIG. 3, which is a schematic diagram of the voltage-controlled characteristic of a MOS when used as a varactor. The gate electrode of the MOS M0 form an end at node Nb1, the source and the drain electrodes of the transistor MO are both connected at node Nb2 to form another end. Then an equivalent capacitor Cb0 is formed between Nb1, Nb2, and M0. FIG. 3 is a relationship schematic diagram between the capacitance of Cb0 and the voltage of Vb12. Where the X-axis is the voltage of Vb12 and the Y-axis is the capacitance of the equivalent capacitor Cb0. As FIG. 3 shows, when the transistor M0 is a voltage-controlled varactor, the voltage-controlled characteristic of it becomes more sophisticated, and more non-linear. Though a small segment of the characteristic curve appears to be linear, it is too narrow to be flexibly used in circuit design.